1. Field of the Invention
The present invention relates to a power regulating system for engine generator, which comprises rectifying AC output of an AC generator, and then reconverting it into an alternating current of arbitrary frequency on an inverter before outputting it.
2. Description of the Prior Art
For example, a device in which a small gasoline engine and a generator are combined to obtain AC power outdoors is known hitherto as a portable engine generator or portable generator, which is used extensively for leisure enjoyment, field works, emergency measures and others. An AC generator such as alternator or the like is used generally as the aforementioned generator, and an engine rotational frequency (n), an output frequency (f) and a generator pole number (p) are interrelated as n=120 f/p, as is known, in this case. Then, a bipolar or quadripolar generator is used generally, and hence for obtaining an AC output stabilized at 50 Hz (or 60 Hz), the engine rotational frequency must be stabilized in a relatively low domain coming in 3,000 rpm (or 3,600 rpm) in the case, for example, of the bipolar generator. Thus, the engine is controlled for rotation by a governor or other means, however, since the engine rotational frequency is in a relatively low domain, the operation efficiency of the engine is low and there is no alternative but to make the generator larger on power capacity. Still further, since the output frequency is set by the engine rotational frequency, it is difficult to keep the output frequency constant all the time.
There is known such engine generator using an inverter power regulating system that will run the engine in a domain high of rotational frequency, convert the output of the generator into a direct current once, and then reconvert it into an alternating current of arbitrary frequency on an inverter before outputting it. The inverter type engine generator has the inverter on an output stage constituted of a semiconductor switching element such as a transistor or the like, and the arrangement is such that an AC output of desired voltage and frequency is obtainable by controlling the switching element on a driving signal (commutating signal) given thereto from an oscillator. Those which are disclosed, for example, in Japanese Utility Model Laid-Open No. 132398/1984, Japanese Patent Laid-Open No. 82098/1985 and Japanese Patent Publication No. 500557/1987 are known as representing such system.
In the generator of the type described above, since the inverter in the output stage is constituted of a semiconductor switching element such as a transistor or the like, it is necessary to provide a circuit for protecting the semiconductor switching element from overcurrent.
However, in the conventional generator of the type described above, a set point for the overcurrent protecting operation is fixedly set in accordance with a particular allowable rated outout. Therefore, when the inverter is to be protected by detecting an overcurrent state of the inverter, it is impossible to reliably protect the inverter as a result of changes in the engine rotational frequency. Accordingly, in order to change the rated output by making the engine rotational frequency variable, it is necessary to provide an external switch or the like, and this complicates the arrangement and necessitates the switch to be actuated to change the rated output, resulting in inferior operability.